Effects of Sodium Dodecyl Sulfate, SDS, Micelles on the Electrochemical Behavior of a Model Arenediazonium Ion. 2. Estimation of the Association Constant of the Electrochemically Generated Aryl Radicals with SDS Micelles

Abstract

The effects of sodium dodecyl sulfate, SDS, on the reductive electrochemistry of p-nitrobenzenediazonium tetrafluoroborate, PNBD, were investigated at two different pHs by employing differential pulse polarography (DPP). At pH ) 2, the reduction peaks of the -N2 + and -NO2 groups are completely overlapped. Upon addition of SDS ([SDS] < critical micelle concentration, cmc), their peak potentials, Ep, are shifted in opposite directions, Ep(-N2 + ) toward more anodic values and Ep(-NO2) toward more cathodic values, and two polarographic peaks are clearly observed, suggesting that PNBD interacts with the sulfate group of SDS leading to the formation of an ion-pair. At the same pH but at [SDS] > cmc, the peak potentials shift in the reverse directions as those below the cmc and the polarographic peaks overlap again, an effect that is interpreted in terms of the incorporation of PNBD to the SDS micellar aggregates. At pH ) 5, the reduction peaks of the -N 2 + and -NO2 are clearly separated. Addition of SDS shifts their peak potentials in opposite directions, the same as those at pH ) 2, up to a maximum (-N2 + ) or minimum (-NO2) at [SDS] ) cmc, after which the directions are reversed getting a constant value. Peak currents, ip, for either -N2 + and -NO2 decrease smoothly reaching a plateau region. Quantitative analyses of the effects of SDS on Ep and on ip at [SDS] > cmc allowed estimations of the association constants of the parent PNBD and of the electrochemically generated aryl radical, Ar • , with SDS micelles. The association constant of the aryl radical is higher than that of PNBD by a factor of ∼2, indicating that the nitrobenzene radicals are preferentially stabilized in SDS micelles. The results obtained, together with those in previous work, suggest that the combination of DPP with arenediazonium ions as probe molecules provides a rapid, easy, and low-cost method to estimate the stability constants of a large number of aryl radicals to biomimetic systems, information that may be valuable for understanding relevant redox reactions in the more complex biological systems.

Micellar Effects on the Reduction of 4‐Nitroimidazole Derivative: Detection and Quantification of the Nitroradical Anion

“…However, our results are contradictory with the previously reported by Mc Intire et al [13] which examined the reductive electrochemistry of nitrobenzene in anionic, cationic and nonionic micelles, and found that the only medium capable of stabilizing the nitroradical anion to the point where it became detectable by cyclic voltammetry was not the cationic, but rather the anionic micelles. The combined results suggest that the influence of micellar systems on the electrochemistry of a particular substrate cannot be predicted only on the basis of simple electrostatic considerations and that micellar effects are strongly dependent upon the substrate and surfactants structures [16]. However, considering that our results show that the nitroradical anion is more persistent in the presence of cationic micelles (Hyamine and CTAB) that in either isotropic aqueous solution or nonionic (Triton-X) or anionic (SDS) micelles an explanation based in electrostatic considerations appear to be more important.…”

Determining α-tocopherol distributions between the oil, water, and interfacial regions of macroemulsions: Novel applications of electroanalytical chemistry and the pseudophase kinetic model

“…A variety of electron donors reduce arenediazonium ions [13,15]. Electrochemical reductions of arenediazonium ions have been studied in aqueous, nonaqueous, and micellar solutions [16][17][18][19][20]. Polarographic techniques were used to estimate partitioning of electroactive compounds [21][22][23], but the models used to fit the results did not explicitly include a separate interfacial region.…”

Spectroscopic studies of arenediazonium ion stability in surfactant solutions

“…It means that the reaction occurs according to the heterolytic mechanism and phenol is its sole product. It follows from literature [10,24,25] that the other two arenediazonium ions decompose according to the same heterolytic mechanism. The absorbances of decomposition products (p-cresol and p-bromophenol respectively) at the characteristic wavelength (278 nm for 4MBD and 292 nm for 4BrBD) are very low in comparison with those of parent arenediazonium ions.…”

“…There are numerous methods of determining the association constant K S [24,25]. Among them, the UV-VIS spectroscopy is very popular.…”

“…The character of microenvironment (its polarity, acidity, ionic strength, etc.) as well as the presence of surfactant aggregates are also of some importance [13][14][15]24,25].…”